The life-long maintenance and regenerative capacity of the hematopoietic system depends upon self-renewal and differentiation of pluripotent hematopoietic stem cells (HSC). The HSC give rise to all mature blood cell types by differentiating through intermediate progenitor cells that undergo lineage commitment and subsequent development along a single pathway (1-5). During the last two decades a highly complex regulatory network of molecular mechanisms, necessary to control lineage commitment and differentiation of blood cells has been identified, including growth factors and receptors, cell-cell interaction molecules, signal transduction molecules and transcription factors (6-15). Due to distinct functional features of HSC, progenitors and mature blood cells, it is reasonable to assume that these properties are regulated at least in part by molecules that are preferentially expressed at particular stages of blood cell development. One approach to identify molecules that are important for self-renewal and lineage commitment of HSC and progenitors is to focus on rare populations of cells that are enriched for HSC and progenitors. Construction of HSC and progenitor cell-specific subtracted cDNA libraries, coupled with cDNA sequencing and microarray-based studies of gene expression patterns, will be necessary to molecularly define self-renewal, functional pluripotency and lineage commitment of HSC and progenitors and to elucidate the extraordinary developmental plasticity of HSC (16-19). Using subtracted cDNA libraries and cDNA microarray approach Phillips et al. (17) have recently reported results of a genomewide gene expression analysis in mouse fetal liver HSC and progenitors. The complete data in the form of a database represent the first step in elucidating the molecular phenotype of hematopoietic stem cells and progenitors.
Elucidation of the differential gene expression during differentiation of hematopoietic stem cell and progenitors should have far reaching implications for ex vivo manipulation of HSC, clinical bone marrow transplantation and gene therapy of hematological disorders.
To identify novel molecules involved in intrinsic regulation of HSC and progenitor cell lineage commitment and differentiation we have generated full-length and subtracted cDNA libraries from mouse adult bone marrow cell populations enriched for HSC (Lin−Sca-1+ cells) and progenitors (Lin−Sca-1− cells) (19). Phenotypically and functionally defined population of primitive Lin−Sca-1+ cells comprises 0.1-0.2% of bone marrow cells and contains virtually all HSC and primitive progenitors, whereas more differentiated Lin−Sca-1− cells contain committed progenitors but lack HSC. Here we describe cloning and characterization of a novel gene, Hepp, that is expressed preferentially in mouse fetal and adult hematopoietic progenitors and mature blood cells.
Certain aspects of the present invention have been disclosed in Abdullah et al. (19).